Priority is claimed on Japanese Patent Application No. 2003-039894, filed Feb. 18, 2003, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a transmitter and a receiver that transmit and receive signals.
2. Description of Related Art
Mobile communication is fundamentally communication beyond the line of sight, and multipath channel is formed by reflection, diffraction, and scattered waves. Furthermore, when the delay time of each path is too large to be disregarded, a multipath channel having delay diffusion. In a multipath channel having delay diffusion, the channel behaves like a particular type of filter and, as a result, transmitted signals thereof receive frequency selective fading (see, for example, “Modulation and De-modulation in Digital Wireless Communication”, by Youichi Satoh, published by the Electronic Information Communication Society 1996, pp. 157-158 and pp. 202-204, and “Mobile Communication”, by Shuuichi Sasaoka, published by Orm 1998, pp. 36-37).
For example, an example of a typical method against frequency selective fading in mobile communication that uses a code division multiplexing (CDM) format is Rake combining. Rake combining is a type of diversity technology, and is a technology that performs diversity (i.e., implicit diversity) using inherence of the signals. A typical method of combining spread diffused signals is the maximal ratio combining (MRC) method. This method combines the received signals with not only compensating the phase distortion but also weighting in accordance with the level of reliability.
In a code division multiple access (CDMA) cellular system, which is a typical mobile communication system, firstly, transmitted signals are spread using inner code in order to increase throughput. In addition, spread signals are also spread using outer code in order to decrease intra-cell and inter-cell interference. Generally, the first spreading uses Walsh code in which the cross correlation is 0 as long as there is no phase difference. The second spreading uses PN code in which the cross correlation characteristic is sufficiently small even under phase difference. PN code has the cross correlation of 1/Np, where the length of PN code is Np (see, for example, “Spread Spectrum Communication Systems”, by Mitsuo Yokoyama, published by Science and Technology Publishing on 1988, pp. 200-203, 401-403, and 523-538).
However, if Rake receiver receives the transmitted signals spread by inner and outer code, then because the cross correlation outer code is not 0, intersymbol interference (i.e., multipath interference) is generated. Furthermore, even if the cross correlation of the inner code is 0, multipath interference increases in proportion with the number of inner code multiplex. Specifically, under two Rayleigh paths with an equal average power channel, the signal to interference power ratio (SIR) when signals that have been spread using Walsh code for the inner code and PN spreading for the outer code are combined using maximal ratio combining at Rake receiver is shown by Formula (1) given below. From Formula (1) it can be seen that the interference power increases in proportion to the number of Walsh multiplex Mw.
                    SIR        =                                            P              1                                                                        M                  W                                                  N                  W                                            ⁢                              P                2                                              +                                    P              2                                                                        M                  W                                                  N                  W                                            ⁢                              P                1                                                                        (        1        )            
In Formula (1), P1 and P2 are power of each path, NW is the length of the Walsh code, and MW is the number of Walsh multiplex.
The increase of the number of the multiplexed code increases the interference power, so Rake combiner is not tolerant to multipath interference with high code multiplexing.
In contrast, orthogonal frequency division multiplexing (OFDM) is known as a multipath interference. Suppression technology OFDM is a multi-carrier transmission system in which sub-carriers are arranged such that the cross correlation between adjacent sub-carriers is 0. The multi-carrier transmission divides the entire bandwidth into narrow bandwidth sub-carriers and signals are transmitted in parallel. Therefore, the throughput in each sub-carrier is reduced and the symbol duration of cash sub-carrier is longer compared with single carrier transmissions. Accordingly, it is possible to make the symbol duration sufficiently longer than the impulse response of the channel, and to reduce the effects of frequency selectivity fading (see, for example, the aforementioned related document “Modulation and De-modulation in Digital Wireless Communication” and also “Frequency Domain Equalization for Single-Carrier Broadband Wireless Systems”, by D. Falconer, S. L. Ariyavisitakul, A. Benyamin-Seeyar and B. Eidson, IEEE Commun. Mag. April 2002, Vol. 40, No. 4, pp. 58-66).
However, in the OFDM system, the peak to average power ratio (PAPR) is high in order to create a signal in a frequency domain. Therefore, the OFDM system reduced its capacity due to the non-linearity of the power amplifier. It has the additional problem that if the carrier frequency is offset by the multipath channel, then the performance is greatly deteriorated.
Therefore, in recent years, Single Carrier with Frequency Domain Equalization (SC-FDE) is proposed as a waveform equalization technology to overcome the problems in the OFDM system. Although the receiver block diagrams of the OFDM and SC-FDE are similar, SC-FDE receiver has an inverse Fourier transformation block and processing other than channel estimation and equalization is performed in a time region. Therefore, the high PAPR and the vulnerability to carrier frequency offset, which are the OFDM problem, can be obviated (see, for example, the aforementioned related documents and also “Frequency Domain Equalization for Single-Carrier Broadband Wireless Systems”, by D. Falconer, S. L. Ariyavisitakul, A. Benyamin-Seeyar and B. Eidson, IEEE Commun. Mag. April 2002, Vol. 40, No. 4, pp. 58-66).
In mobile communication that uses a CDM system, for example, the number of Walsh multiplex MW is small, the diversity gain of Rake combiner are greater than deterioration caused by multipath interference. Therefore, the Rake combiner is better than SC-FDE with low code multiplexing. Specifically, a computer simulation result is shown in FIG. 10.
FIG. 10 shows PER of Rake combining and SC-FDE dependence of Eb/N0 under two Rayleigh paths with an equal average power.
As is shown in FIG. 10, the multipath interference of Rake combiner increases by increasing the number of Walsh multiplex MW. When the number of Walsh mutiplex MW is 8 or more, PER of SC-FDE is smaller than Rake combiner. However, when the number of Walsh multiplex MW is 4 or less, conversely, the PER characteristics of Rake combiner are better. This is because while Rake combiner combines the power of the each path to increase the desired signal.
Accordingly, when reliable communication is required than high throughput, SC-FDE performance is less advantageous than Rake combiner. Therefore, SC-FDE will be difficult to obtain a satisfactory communication quality.
In this way, in a transmitter and receiver of a conventional CDM system, it has been difficult for single receiving method to obtain a satisfactory communication quality for a variety of channel conditions.
Note that in the simulation shown in FIG. 10, as is shown in FIG. 3A, the frame structure of SC-FDE thereof is formed by unique words (UW), which are pilot signals, and by data. The UW is constant amplitude zero auto-correlation (CAZAC) sequence of 64 chips, and a cyclic prefix (CP) of 16 chips is inserted at the head of both the UW region and the data region. As is shown in part FIG. 3B, the frame structure of Rake combining thereof is formed by pilot signals and data signals, and the pilot signals are BPSK signals of 96 chips, which is “1”.                Moreover, complex scrambling (for example, “3G Wireless Technology Workshop Part TIA/EIA-95 CDMA, cdma2000, HDR,” pp. 29-34, January 2001) is not performed to the data of SC-FDE, and the channel estimation is ideal. In addition, minimum mean square error frequency domain equalization (shown, for example, in “Low Overhead Pilot-Aided Synchronization for Single Carrier Modulation with Frequency Domain Equalization”, Proc. GLOBECOM '98, pp. 2068-2073, Sydney, Australia, November 1998) is performed. On the other hand, the data of Rake combining is transmitted after complex scrambling using complex PN code of 1024 chips.        
TABLE 1 shows the remaining simulation parameters. Moreover, the simulation knows the arrival time of the each path, at the timing of the preceding wave. Rake receiver combines each path with maximal ratio combining.
TABLE 1ItemContentsModulationQPSKSpreading1, 4, 8, 16 multiplex by 16 array walsh codeChip duration1/1.2288 [μsec]ChannelTwo rayleigh paths with equal average powerMaximum dropper60 [Hz]frequencyDelay time1 chip duration